Fix cluster verification issues on multi-group clusters
Fix cluster verification issues on multi-group clusters

This patch attempts to fix a number of issues with “gnt-cluster verify”
in presence of multiple node groups and DRBD8 instances split over nodes
in more than one group.

- Look up instances in a group only by their primary node (otherwise
  split instances would be considered when verifying any of their node's
  groups)
- When gathering additional nodes for LV checks, just compare instance's
  node's groups with the currently verified group instead of comparing
  against the primary node's group
- Exclude nodes in other groups when calculating N+1 errors and checking
  logical volumes

Not directly related, but a small error text is also clarified.

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

apyrgio/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,leshchevds/ganeti,yiannist/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,ganeti/ganeti,andir/ganeti,yiannist/ganeti,bitemyapp/ganeti,leshchevds/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,mbakke/ganeti,onponomarev/ganeti,yiannist/ganeti,mbakke/ganeti,grnet/snf-ganeti,andir/ganeti,onponomarev/ganeti,ganeti/ganeti,andir/ganeti,leshchevds/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,grnet/snf-ganeti